Lemma 2  [1]} If \(A=(a_{nk})_{n,k=0}^{\infty }\) , then
\(A\in (c,c)\)  if and only if conditions (REF ), (REF ) holds and
\( \lim _{n\rightarrow \infty } A_n=\lim _{n\rightarrow \infty } a_{nk}\hspace{28.45274pt} \text{exists for all } k\) 

